At present, matrix materials for an absorbable implanted medical device mainly include polymers, a magnesium-based alloy and an iron-based alloy. The most frequently applied polymer is polylactic acid, which can be completely degraded and absorbed, with degradation products of carbon dioxide and water, but its mechanical property is poor. The size of the polymer-based device should be larger than the metal-based device so that the polymer-based device has the same mechanical property as the metal-based device, which limits application of the polymer-based device. The magnesium-based alloy and the iron-based alloy have advantages of ease in processing and molding, and high mechanical strength. However, as the magnesium-based alloy corrodes too quickly in a human body, it is necessary to enlarge the size of a magnesium-based alloy device to obtain the mechanical property in the early stage of implantation, and this also limits the application of the magnesium-based alloy device.
In terms of clinical application, when the absorbable implanted medical device fulfills its expected use, after a diseased portion is cured and is recovered to its normal shape and function (cured), on the premise of not causing a new biological compatibility problem, the less time needed for the device to be completely degraded and absorbed by an organ the better. According to different portions to which the device is clinically applied, the recovery period is generally considered as 1 to 6 months, and within this period of time, the device is required to keep a structural integrity and have a sufficient mechanical property. When used as an implanted medical device matrix material, the iron-based alloy has a good biological compatibility, and iron ions contribute to inhibiting smooth muscles and promoting growth of endothelial cells, but due to the slow corrosion of the iron-based alloy in the body, the iron-based alloy device would not be corroded completely until a long time after the diseased portion is cured; and therefore, it is necessary to accelerate corrosion to shorten the corrosion cycle of the iron-based alloy.
A research has shown that if the surface of the iron-based alloy is coated with a degradable polyester coating, its corrosion speed would be increased. Degradation of the degradable polyester coating in the body would lower the pH value of a local microenvironment near a device implantation position, thereby forming a local micro acidic environment where the iron-based alloy is corroded faster to generate iron salt and/or iron oxides and/or iron hydroxides serving as corrosion products.
For the iron-based alloy device of a predetermined specification, the corrosion speed of the iron-based alloy and whether the iron-based alloy is finally completely corroded or not are determined according to the amount of the degradable polyester coating and the type and the nature of degradable polyester. Under conditions that the type and the nature of the degradable polyester have been selected and the amount of the degradable polyester is sufficient to completely corrode an iron-based alloy substrate, extremely high corrosion speed or local severe corrosion of the iron-based alloy would affect the structural integrity and the mechanical property of the iron-based alloy device in the early stage of implantation (1 to 6 months, namely the above-mentioned recovery period), thereby it is difficult for the device to meet a requirement for clinical application. These defects are specifically as follows: (1) a degradation product of the degradable polyester coating is acidic, and there are small molecular residues with a higher degradation speed in degradable polyester (for example, the standard monomer residue amount of the polylactic acid is less than 2%), that will result in faster corrosion of the iron-based substrate in the early stage of implantation, for example, after the device is implanted into a coronary artery for about to 7 days, excessively fast corrosion and accumulation of the corrosion products cause incomplete endothelialization of the inner surface of the device, which increases a risk of acute thrombosis and subacute thrombosis; and (2) the heterogeneity of degradable polyester degradation easily leads to non-uniform corrosion of the iron-based alloy substrate, and local fast corrosion possibly results in breakage, which leads to a fact that it is hard for the iron-based alloy substrate to meet requirements on a structural integrity and a mechanical property in the early stage. Although the excessively fast corrosion of the iron-based alloy device in the early stage of implantation can be prevented by reducing the amount of the degradable polyester coating, the corrosion cycle of the iron-based alloy device would be prolonged. Therefore, for the iron-based alloy device including degradable polyester, under the conditions that the type and the nature of the degradable polyester and a ratio of the amount of degradable polyester to the iron-based alloy are determined, it is necessary to seek a way to reduce the early corrosion speed of the iron-based substrate in the acidic environment which is generated by the degradable polyester to guarantee the mechanical property of the device in the early stage of implantation.